1. Field of the Invention
The present invention relates to a muzzle brake vibration absorber for use on a gun barrel. More particularly the muzzle brake vibration absorber is used to store potential energy during gun firing and re-introduce the energy, in part, to the gun barrel out of phase relative to the gun barrel motion. This results in the firing deviation of the gun barrel being mitigated. Most particularly, the muzzle brake vibration absorber also may function as, or in combination with, a muzzle brake or fuze setting device for the gun system.
By improving the accuracy of the gun system, the gun system provides a greater operational efficiency through greater accuracy. This decreases the logistical burden to resupply ammunition to operational combat units.
2. Brief Description of the Related Art
Numerous attempts have been made to improve the accuracy of gun weapon systems, particularly those guns systems that are subject to vibrational disturbance during firing. With gun systems becoming increasingly accurate, the affect of vibrational disturbances on gun accuracy has become more pronounced. Additionally, the use of longer and more slender gun barrels, such as that on the XM291 tank gun system, for providing higher projectile exit velocities increases the occurrence of flexural vibrations within the gun systems.
Several methods have been used to improve the accuracy of gun weapon systems.
One method includes the extension of the gun mount/cradle of the gun systems. One means of reducing the receptance of a gun barrel to flexural vibrations is to decrease the effective cantilevered length of the gun system. This may be achieved by increasing the length of the supporting structure that holds the gun barrel which effectively increases the ratio of stiffness to inertia of the system. The square of the ratio of stiffness to inertia is indicative of the resistance of a gun barrel to low frequency vibrations.
A variation on the extended mount approach has been to utilize a traditional mount to support the gun barrel, and to incorporate damping pads through a mount extension that couples the barrel to the cradle with low stiffness and high damping. The result is that the mount extension need not be as solid, since increased stiffness is not the primary objective of the approach. An example of this approach is the British 30 mm, L21A1, system commonly called the RARDEN. (see Geeter et al., xe2x80x9cLow Dispersion Automatic Cannon System (LODACS) Final Report (U),xe2x80x9d ARDEC Technical Report ARSCD-TR-8201 1, Picatinny Arsenal, New Jersey, August 1982).
Although the extension of the gun mount/cradle may succeed in reducing vibrations, it can present a negative impact of increasing the imbalance of several gun weapon systems, since the center of gravity of typical gun systems is forward of the trunnion bearings. This imbalance necessitates the application of control torques, equal and opposite to the weight of the gun weapon system, multiplied by the horizontal offset of the center of gravity from the pivot point. These requirements places a heavy burden on the pointing system.
Further, for many gun systems extension of the gun mount/cradle becomes ungainly as the ratio of in-mount barrel length to overall barrel length increases. Packaging such support structures in a fielded weapon system becomes difficult.
Another approach has been the incorporation of thicker gun barrels. Gun barrels may be constructed with thicker walls. Since the stiffness is a function of the outer radius to the fourth power, and the inertia is a function of the outer radius to the second power, significant increase to the ratio of stiffness to inertia of the system can be made.
Thicker gun barrels increase the ratio of stiffness to inertia, but require a significant ratio between the inner radius (the radius of the bore) and the outer radius. If the wall thickness, that is the difference between the inner and outer radii, is reasonably small relative to either radius, a thin walled approximation would have the inertia and stiffness increase proportionally to each other, thus no net gain. For example, a Taylor series expansion of the ratio of stiffness to inertia as a function of outer diameter is dominated by the linear term for barrels whose wall thickness is a fraction of the bore radius. The second term exists, but it doesn""t dominate until the wall thickness becomes impractical.
A related problem with this approach is that increased weight of the barrel is a direct consequence. This exacerbates both the extension of the center of gravity of the gun further out from the trunnions, and increases overall weapon weight which is supposed to be minimized.
A composite barrel construction is another approach. Gun barrels may be constructed of materials with a higher stiffness to inertia ratio, such as carbon fiber reinforced epoxy, or composite over-wraps of traditional gun steel barrels. The goal is to increase the net ratio of stiffness to inertia of the system, and this can be achieved. This is discussed in Hasenbein, et al., xe2x80x9cMetal Matrix Composite-Jacketed Cannon Tube Program,xe2x80x9d ARDEC-Benxc3xa9t Technical Report ARCCB-TR-9 1027, Watervliet Arsenal, NY, August 1991.
Composite barrel construction is a viable means to enhance the structural stability of gun weapon systems. It is, however, challenged by the need to protect the barrel from the hot and erosive action of the propellant gases. This typically results in a composite over-wrap incarnation over a thin-walled steel barrel. The remaining challenge is to maintain the bond between the base material and the composite over-wrap during both manufacture, especially the autofrettage process, and the firing loads which create concurrent radial dilation of the barrel and axial recoil loads. This firing dynamic challenge is exacerbated by the pressure discontinuity that travels behind the obturation of the projectile with a speed that may resonant a traveling flexural wave of the bore surface. Other challenges include impaired heat transfer across the insulting composite and increase recoil velocity of the cannon during operation.
Fluted gun barrels also have been used. Gun barrels may be constructed with flutes that look like fins emanating from the center of the gun. In analogy with design of an I-Beam the general design concept is to get the steel at a greater radius for an increased stiffness; without increasing the inertia in proportion. An example of this approach is the British 30-mm, L21A1 system commonly called the RARDEN. (See Geeter, et al., xe2x80x9cLow Dispersion Automatic Cannon System (LODACS) Final Report (U),xe2x80x9d ARDEC Technical Report ARSCD-TR-;82011, August 1982). However, fluted gun barrels are expensive to manufacture, increase system weight, and compromise a desirable static stress distribution that is manufactured into most large caliber gun barrels using a process called autofrettage.
The application of active controls and feed-forward cancellation has been used. If the input excitation can be anticipated, a control signal can be applied through an actuation system to preempt the disturbance energy. An example for a tank gun system while traversing rough terrain would be the use of a sensor to detect vertical acceleration of the tank hull, and apply immediate counteraction force through the elevation actuator system. In many tank guns the center of gravity extends forward of the trunnion bearings. This is a result of the limited working volume within the armor protected turret. Thus, a vertical heave upwards applies a torque to the gun system that may be cancelled by an applied downward force at the elevation coupling, behind the trunnions. For current systems, feed-forward cancellation treats the gun barrel as a rigid body, and ignores flexural modes, and in particular the first flexural mode.
The concept behind active feed-back vibration cancellation is to sense the vibrations of the structure under control, both amplitude and phase, and to apply control forces to the structure to cancel the detected vibrations. This requires both sensors, actuators, and the design of a stable control law; a means to determine what load to apply based on sensor information and a priori knowledge of the dynamic behavior of the system.
Active feed-back vibration cancellation has fundamental problems with structural control. The partial differential equations that govern the vibrations of continua are termed stiff. In this context xe2x80x9cstiffxe2x80x9d implies that structures contain many natural modes of vibrations with a wide variation in the time-constants or frequencies of response. Thus, although a gun barrel may be dominated by its first mode, on the order of 20 Hz for a tank gun system, it posses vibratory modes with fundamental frequencies orders of magnitude higher. The result of this is that the speed of response required of any active control system is high, and may become impractical.
Additional challenges to feed-back vibration cancellation are stability related. Fundamentally, this type of active control attempts to cancel vibration energy with high force input to the structure. Relatively small discrepancies in the sensors and actuation can result in adding vibrational energy to the structure. This energy often collects in vibratory modes that were not included in the control formulation, particularly that, as a xe2x80x9cstiffxe2x80x9d system there are many natural modes. Thus, the vibration energy may not even be seen by the sensor system, or may migrate to frequencies that are too high for the actuation system.
Yet another challenge with this feed-back vibration approach is that the free-end of the gun barrel exhibits the most vibration; it is the anti-node of the structure and yet it is removed from control forces by the cantilevered barrel length. From the perspective control system design theory the implication of this is that the system exhibits xe2x80x9cnon-minimum-phasexe2x80x9d behavior. This behavior limits the so-called control gain that may be applied to the system because high gains may drive the system unstable. In other terms, the controlled system exhibits right-hand Laplace plane zeros. These zeros cause the locus of system poles to cross the imaginary axis from the left-hand-plane to the right as the feedback gain is changed. Once in the right hand plane, a pole drives the system unstable with ever increasing amplitude.
Smart structures also may be used. Similar to the feed-back vibration cancellation technology previously described, smart structures include both actuation and sensor transducers to reduce-control vibrations within the structure itself. In the case of a gun barrel, a smart structure approach would entail the coupling of sensor and control mechanisms along the cantilevered span of the barrel. The main difference with the feed-back control method is that the dynamic system of the gun structure itself is changed. Additionally, smart structures tend to be expensive and difficult to manufacture, especially for a gun barrel shock and vibration environment.
In view of the foregoing, there is a need for improvements for increasing the accuracy of gun systems. The present invention addresses this and other needs.
An object of the present invention is to provide a muzzle brake vibration absorber for use on a gun barrel to increase the accuracy of the gun system.
The present invention includes a muzzle brake vibration absorber for a gun weapon system comprising a vibration absorber mass and means for storing potential energy between the vibration absorber mass and a free end of a gun barrel effective to provide an elastic coupling therebetween.
The present invention also includes a method for stabilizing a gun weapon system for firing comprising the steps of providing a muzzle brake vibration absorber on a gun weapon system having a vibration absorber mass and means for storing potential energy between the vibration absorber mass and a free end of a gun barrel effective to provide an elastic coupling between the mounting collar and the vibration absorber mass and firing the gun weapon system, wherein the vibrational disturbance of the gun decreases.
Additionally, the present invention includes a stabilized gun weapon system product made from the process comprising the steps of providing a muzzle brake vibration absorber on a gun weapon system having a vibration absorber mass and means for storing potential energy between the vibration absorber mass and a free end of a gun barrel effective to provide an elastic coupling between the mounting collar and the vibration absorber mass and firing the gun weapon system, wherein vibrational disturbance within the gun decreases.
Other and further advantages of the present invention are set forth in the description and appended claims.